1. Field of the Invention
The present invention relates to a laser irradiation method of annealing a semiconductor film using a laser beam (hereinafter referred to as laser annealing) and a laser irradiation apparatus for performing the laser annealing (apparatus including a laser and an optical system for guiding a laser beam output from the laser to a member to be processed). Further, the present invention relates to a semiconductor device manufactured by the steps including the laser annealing step and a method of manufacturing the semiconductor device. Note that the semiconductor device mentioned through the specification includes an electro-optical device such as a liquid crystal display device or a light emitting device and an electronic device including the electro-optical device as its component.
2. Description of the Related Art
In recent years, techniques of crystallizing a semiconductor film formed on an insulating substrate such as a glass substrate or improving the crystallinity thereof by performing laser annealing have been widely studied. Silicon is often used for the semiconductor film.
The glass substrate is at low cost and has a lot of processability in comparison with a synthetic quartz glass substrate that has been conventionally used in many cases, and has an advantage that it easily enables the manufacture of a substrate with large area. This is a reason that the above-mentioned study is made. Further, the reason a laser is used in crystallization from choice is that the melting point of the glass substrate is low. Laser can give high energy only to the semiconductor film without largely raising the temperature of the substrate.
Since a crystalline semiconductor is formed of a large number of crystal grains, it is also called a polycrystalline semiconductor film. The crystalline semiconductor film formed by performing laser annealing has high mobility, and thus, a thin film transistor (TFT) is formed using the crystalline semiconductor film. For example, the TFT is widely used for a monolithic liquid crystal electro-optical device or the like in which TFTs for a pixel portion and for a driver circuit are formed on a glass substrate.
Further, the method, in which a pulse laser beam of an excimer laser or the like with large output is processed in an optical system so as to have a shape of a spot of several by several centimeters square or a linear shape of 10 cm length or more on an irradiation surface, the laser beam is scanned (or the irradiation position of the laser beam is made to move relatively to a surface to be irradiated), and laser annealing is performed, attains high mass production and is excellent from the industrial viewpoint. Thus, the method is used from choice.
In particular, when the linear beam is used, laser irradiation can be conducted over the entire irradiation surface with scanning only in a direction perpendicular to the longitudinal direction of the linear beam, which is different from the case where the spot laser beam that requires scanning in the front and rear directions and in the right and left directions is used, which leads to high mass production. The reason the scanning is performed in the direction perpendicular to the longitudinal direction is that the direction is most effective. Owing to the high mass production, the laser annealing method in which the linear laser beam that is obtained by processing a pulse oscillation excimer laser beam in an appropriate optical system is used currently becomes the main stream of a manufacturing technique of a liquid crystal display device using a TFT.
However, the crystalline semiconductor film manufactured by the laser annealing method is formed from a plurality of crystal grains, and the position and the size of the crystal grains are random. The TFT manufactured on the glass substrate is formed by separating the crystalline semiconductor film with island-like patterning in order to realize element isolation. In this case, the crystalline semiconductor film can not be formed with the designation of the position and the size of the crystal grains. There exist numerous recombination centers and trapping centers which derive from an amorphous structure, a crystal defect or the like exist in an interface of the crystal grain (crystal grain boundary) compared with the inside of the crystal grain. It is known that when a carrier is trapped in the trapping center, the potential of the crystal grain boundary rises, which becomes a barrier to the carrier, whereby the current transporting property of the carrier is lowered. The crystallinity of the semiconductor film in a channel forming region has a great effect on the characteristic of the TFT. However, it is hardly possible that the channel forming region is formed of a single crystal semiconductor film without influence of the crystal grain boundary.
Further, it is known that growth distance of the crystal grain is in proportion to the product of a crystallization time by a growth rate. Here, the crystallization time indicates the time until the completion of crystallization of a semiconductor film from the generation of a crystal nucleus in the semiconductor film. Further, assuming that the time until the completion of crystallization from melting of the semiconductor film is a melting time, if a cooling rate of the semiconductor film is made gentle by extending the melting time, the crystallization time becomes longer. Thus, the crystal grain of a large grain size can be formed.
In order to form the channel forming region by the single crystal semiconductor film without influence of the crystal grain boundary, various attempts for forming the position-controlled crystal grain of large grain size are made in the laser annealing method. First, a solidifying process of the semiconductor film that has been subjected to irradiation of a laser beam is explained.
It takes time in a degree until solid phase nucleation occurs in a liquid semiconductor film that has been completely melted by laser beam irradiation. Numberless and uniform (or nonuniform) nucleations occur and grow in a completely melted region, whereby the solidifying process of the liquid semiconductor film is completed. Obtained in this case are crystal grains which are random in position and size.
Further, in the case where the semiconductor film is not completely melted by the laser beam irradiation and solid phase semiconductor regions remain partially, crystal growth begins at the solid phase semiconductor regions immediately after the laser beam irradiation. As described above, it takes time in a degree until the nucleation occurs in the completely melted region. Thus, a solid-liquid interface, which is the tip of the crystal growth, moves in a horizontal direction to the surface of the semiconductor film (hereinafter referred to as lateral direction) until the nucleation occurs in the completely melted region, whereby the crystal grain grows several tens of times as long as the film thickness. This growth ends with the occurrence of numberless and uniform (or nonuniform) nucleations in the completely melted region. Hereinafter, this phenomenon is referred to as a super lateral growth.
In an amorphous semiconductor film or a polycrystalline semiconductor film as well, an energy region of a laser beam where the super lateral growth is realized, exists. However, the above-mentioned energy region is very narrow, and the position where a large crystal grain is obtained can not be controlled. Further, the regions except for the region of the large crystal grain are microcrystalline regions where numerous nucleations occur or amorphous regions.
As described above, if a temperature gradient in the lateral direction can be controlled (a heat flow is made to occur in the lateral direction) in the laser beam energy region in which the semiconductor film is completely melted, the growth position and the growth direction of the crystal grain can be controlled. Various attempts are carried out in order to realize this method.
For example, James S. Im et al. of Columbia University show a sequential lateral solidification method (hereinafter referred to as SLS method) in which a super lateral growth is realized at an arbitrary location. In the SLS method, a slit-shape mask is shifted every shot by a distance (approximately 0.75 μm) in which the super lateral growth is conducted to perform crystallization.
Further, Matsumura, M. et al. of Tokyo Institute of Technology announced a method of forming position-controlled crystal grains of large grain size in the 47th Applied Physics Association Lectures. In the method, an insulating layer of which an upper surface has a square shape is embedded in an amorphous silicon film, and an insulating film is formed on the amorphous silicon film. In conducting irradiation of a laser beam, energy of the laser beam is made to have a gradient by using a phase shift mask, and the portion above the insulating layer is irradiated with the laser beam with low energy. That is, the amorphous silicon film below the insulating layer is cooled most fast and crystal nuclei are generated there after the irradiation of the laser beam because of a light shielding effect of the insulating layer and the energy gradient due to the phase shift mask. On the other hand, since the amorphous silicon film of other regions is still in a melted state, the crystal nuclei grow to the melted region and the crystal grains of large grain size which are position-controlled are formed.
There are various kinds of laser beams, and in general, crystallization is conducted using a laser beam of which a light source is a pulse oscillation type excimer laser (hereinafter referred to as excimer laser beam). The excimer laser has an advantage that it has large output and repeat irradiation with high frequency is possible. Further, the excimer laser beam has an advantage that an absorption coefficient to a silicon film is high.
KrF (248 nm wavelength) or XeCl (308 nm wavelength) is used as an excitation gas in order to form the excimer laser beam. However, gases such as Kr (krypton) and Xe (xenon) are very costly. Thus, there is a problem in that an increase of manufacturing cost is incurred with the high frequency of gas exchange.
Further, an exchange of attached devices such as a laser tube for conducting laser oscillation and a gas refining device for removing an unnecessary compound generated in an oscillation process is needed once in two to three years. Many of these attached devices are expensive, and there is also a problem in that the increase of manufacturing cost is incurred.
As described above, a laser irradiation apparatus using the excimer laser beam has high performance, indeed. However, the maintenance involves a lot of trouble, and also, there is a defect that the laser irradiation apparatus has high running cost (here, the running cost indicates the cost generated with operation) as a laser irradiation apparatus for mass production.
Then, in order to realize a laser irradiation apparatus with running cost lower than that of the excimer laser and a laser annealing method using the laser irradiation apparatus, there is a method of using a solid laser (a laser that outputs a laser beam with a crystal rod as a resonant cavity).
The reason for the use of the method is that the present solid laser has large output but has a very short output time. There are LD (laser diode) excitation, flush lamp excitation and the like as excitation methods of the solid laser. A large current needs to be flown through the LD in order to obtain large output by the LD excitation. Therefore, the life of the LD becomes short, and as a result, the LD excitation costs highly in comparison with the flash lamp excitation. Because of the above reason, most of solid lasers for the LD excitation have small output, and are in the development stage for attaining industrial lasers with large output in the present situation. On the other hand, a flash lamp can emit very strong light. Thus, the laser excited by the flash lamp has large output. However, in the oscillation by the flash lamp excitation, electrons excited by instantaneously input energy are released all at once, and thus, the laser output time becomes very short. As described above, the present solid laser has large output but has the very short output time. Therefore, it is difficult that the formation of the crystal grain, of which the grain size is equal to or larger than that of the crystal grain formed by laser crystallization using the excimer laser, is realized by laser crystallization using the solid laser. Note that the output time indicates half-width in one pulse through this specification.
Here, crystallization of a semiconductor film is conducted using a YAG laser that is a typical solid laser. The YAG laser with flash lamp excitation is used. The laser beam of the YAG laser is modulated into second harmonic by a nonlinear optical element, and then, the second harmonic is processed to have a linear shape of 10 cm length or more in an optical system to thereby irradiate a silicon film. The grain size of the crystal grain formed by laser annealing using the YAG laser has been much smaller than that of the crystal grain formed by using the excimer laser. The state of the crystal grains formed using the YAG laser is shown in FIG. 6. When a TFT is manufactured using a crystalline semiconductor film having the above crystal grain, a large number of crystal grain boundaries exist in a channel forming region that has an important influence on the electrical characteristic of the TFT, which becomes a factor in reduction of the electrical characteristic. As described above, the fact that the present solid laser has large output but has a very short output time can be given as the reason that only a small crystal grain is formed by laser annealing using the solid laser. Further, as another reason, the fact is given that only the energy density lower than that suitable for crystallization is obtained in case of the linear shape of 10 cm length or more. Of course, as the countermeasure against this case, it is considered that laser annealing is performed using the laser beam condensed to the energy density suitable for crystallization. However, it is desirable that the laser annealing with the YAG laser is conducted with at least approximately the same process efficiency as the laser annealing with the excimer laser. In order to achieve this, it is preferable that a laser beam is processed into the linear beam of which the length is approximately equal to or larger than that of the laser beam of the excimer laser.
Further, in the SLS method, a precise control of micron order is needed for the technique of relative positioning between a mask and a substrate, and thus, the complicated laser irradiation apparatus is required in comparison with normal one. Further, there is a problem on throughput in the case where the SLS method is used in manufacturing a TFT applied to a liquid crystal display having a large surface area.
Moreover, in the method announced by Matsumura et al., a phase shift mask for making an energy gradient of a laser beam needs to be used. Therefore, the precise control of micron order is needed for the technique of relative positioning between the phase shift mask and an embedded insulating layer, and also, the complicated laser irradiation apparatus is required in comparison with normal one.